Computer Aided Design and Drafting (CADD) has become a standard design tool for manipulating and recording design information in many industries including construction and manufacturing. CADD models are used to accurately depict the position, shape and orientation of objects, and their relationship to other objects, within a defined environment. The ability to model the objects with the defined environment greatly reduces the amount of engineering effort that must be expended in devising a working design that can be efficiently and economically implemented.
U.S. Pat. Nos. 5,100,229 and 5,110,202, the contents of both of which are incorporated herein by reference, for example, describe systems and methods in which a CADD model is used in connection with a portable position sensor and at least one referent station to identify specific points within an environment. The position information can be used to control construction equipment or model different variations of proposed designs based on the actual measured environment. U.S. patent application Ser. No. 07/714,891 further describes a system for gathering spatial and descriptive data on existing structural elements within an environment that can be used to build CADD models. In order to be commercially viable, however, the position measurement systems described above must be manufactured in an efficient an economical manner that permits their implementation on a wide scale with a high degree of reliability.
It is an object of the invention to provide improvements in the transmitter units and receiver units of the basic systems described above, in order to make the systems easier and less expensive to manufacture while maintaining a high degree of reliability.